The Phoenix-class is 2.5 km long,[1] making it one of the largest known UNSC ships in the organization's naval history to lack the official designation of a carrier-class vessel.

The Phoenix-class ships were designed to transport thousands of colonists from Earth, or one of the other Inner Colonies, as well as terraforming equipment to alter the existing atmosphere and/or biosphere to become compatible with human habitation. Once the terraforming is completed, the colonists settle the planet. On at least one instance, a colony ship was decommissioned to aid in the development of a colony. On Harvest, the Skidbladnir's fusion reactor was used to power the first settlement, Utgard and its data processing storage housing the Agricultural Operations AI, Mack was also utilized. The ship's AI, Loki, became the Planetary Security Intelligence, sharing a core with Mack, covertly monitoring the colony for trouble for the UNSC.[6] Whether this was a common fate for colony ships during the colonization of other worlds is unknown.

At least one Phoenix, Spirit of Fire, was modified for wartime use by the United Nations Space Command and redesignated a Phoenix-class support vessel.[7] The refitted ships carry vehicles, troops, and supplies as support for ground forces. It also carries the resources necessary to build firebases, with the base structures being repurposed colony starter units.[8]

Once converted, a Phoenix-class ship becomes an extremely effective troop transport and orbital support ship, with the ability to carry large amounts of ground troops and ample supplies to sustain them in the field with little to no external contact for considerable amounts of time. It is also able to utilize its deck guns, which appear to be scaled-down MAC guns, to rapidly conduct multiple "MAC-blasts" in support of ground troops.[9][note 1] The ship plays a vital role in battle by dispatching D82-EST Darter dropships to quickly and efficiently deploy prefabricated firebases. However, despite the conversion of the Phoenix-class from a colony transport ship to a warship, it is still lacks to capacity to operate as a front-line ship to ship combatant; the super-structure of the is vulnerable to light pulse laser fire, and despite the addition of armor plating, the outer edges of Spirit of Fire's hull were ripped away when the ship collided with a CPV-class heavy destroyer.

The phoenix was a mythological bird which would be reborn at the end of its lifetime; in a similar fashion, the Phoenix-class colony ship would be decommissioned and stripped down upon colonizing a planet, the ship's materials "giving birth" to a new colony.

The name Phoenix may be a reference to Bungie's canceled game of the same name,[8] or, more likely, the name is derived from an Ensemble prototype which pitched humans against invading Martians in a War of the Worlds style, which was titled Phoenix.[10]

^All Halo Wars media, including the in-game timeline, the official website, and Halo Wars: Genesis, state that the Spirit of Fire was outfitted with a single MAC during its military refit. During the cutscene prior to the level Dome of Light, Captain Cutter asks, "Can we use the ship's cannons to take it out directly?" At the end of this level, the player is able to request forty MAC rounds in very rapid succession, which is impossible for a standard MAC; throughout most of the game, the player can request up to four MAC rounds simultaneously. In addition, the rounds released by the "MAC Blast" power cause very limited damage. Thus, it stands to reason that these weapons were used to provide orbital bombardment by the Spirit of Fire rather than its primary MAC, which would cause great collateral damage and would have a considerably lower rate of fire